The invention relates generally to the processing of photosensitive material.
More particularly, the invention relates to the processing of an elongated band of exposed and developed photosensitive material.
An exposed and developed photographic filmstrip generally has a series of images which are situated next to one another as considered longitudinally of the filmstrip. Such a filmstrip is usually provided with a longitudinal row of perforations adjacent to its edges and the perforations can be engaged by sprocket wheels in order to advance the filmstrip. Frequently, there is no specific positional relationship between the images and the perforations.
In photographic laboratory apparatus, it is occasionally necessary to photoelectrically determine the positions of the images in order to control various operations. By way of example, the positions of the images must be determined when the edges of 135 mm film are to be notched adjacent to the images. The notches may serve, for instance, to position the film in a printing station and then in a cutting station. It is also possible to position the images in the printing station directly on the basis of the photoelectric scan and to perform another such scan for the subsequent cutting operation. The photoelectric scan usually identifies an edge of an image, e.g., the leading edge, by the abrupt transition in transparency which occurs at an edge.
In all of the above cases, the location at which the filmstrip is scanned differs from that at which an operation is performed on the filmstrip. The reason is that, after a transparency transition indicative of an image edge has been found, it is of advantage to investigate another section of the filmstrip in order to ascertain that the transparency transition is not due to an object, e.g., a vertical and poorly illuminated telephone pole, which causes a transparency transition similar to an edge. A reasonably positive identification of an image edge is accordingly possible only after scanning an additional section of the filmstrip which includes at least one image. A more positive identification is achieved when the additional section includes several images. Thus, in order to identify an image edge with an adequate degree of certainty, the scanning station must be spaced from the following operating station by a distance which at least equals the length of an image, i.e., the distance between the leading and trailing edges of an image, and preferably equals several such lengths.
Spacing of the scanning station from the operating station requires very accurate monitoring of the distance travelled as the filmstrip advances from the scanning station to the operating station. Conventionally, measurement of the distance travelled is carried out using rollers which fractionally engage the filmstrip and are arranged to generate pulses as they rotate. For the measurement to be precise, no slip should occur between the filmstrip and the rollers and, in addition, the diameters of the rollers should not change due to wear. However, it is difficult to satisfy both of these requirements simultaneously. Rollers having a high coefficient of friction are generally subject to wear and can also deform elastically. On the other hand, rollers which do not wear easily have a smooth surface so that slip takes place more readily. Accordingly, accurate monitoring of film travel, which is especially critical for larger distances, cannot be achieved with this measurement system.